120-kilogram class ultra-high-strength galvanized steel sheet and method for producing the same

Abstract

The present invention provides a 120-kilogram class ultra-high strength galvanized steel sheet excellent in resistance spot weldability, which contains C: 0.18 to 0.24%, Mn: 2.3 to 3.0%, Si: 0.5 to 1.7%, Al: 0.02 to 1.0%, 0.55 < Si + Al ≤ 1.75%, C + Si / 30 + Mn / 20 ≤ 0.395%, and at least one of Nb, Ti, B, Cr, Mo, REM, and the balance is Fe and inevitable impurities. However, taking the thickness of the steel sheet as t, the resistivity of the steel sheet is 0 < R1 ≤ 55 μΩ·cm, and from the interface between the plating layer and the steel matrix, the resistivity of the steel sheet in the range of 0.025t or more to 0.05t or less in the steel matrix direction is 0 < R2 ≤ 15 μΩ·cm, and the resistivity of the material in the range of 0.01t or more to 0.015t or less is 0 < R3 ≤ 35 μΩ·cm, and 1.5R1 1 / 2 - 0.1R2 - 0.25R3 > 0 is satisfied.

Description

Technical Field

[0001] The present invention relates to an ultra-high strength galvanized steel sheet and a method for manufacturing the same, and particularly to a 120-kilogram class ultra-high strength galvanized steel sheet excellent in resistance spot weldability and a method for manufacturing the same.

Background Art

[0002] In the context of the macro policy of "carbon neutrality and carbon peaking", more efficient energy conservation, emission reduction, and consumption reduction have become important goals for the automotive industry. The lightweighting of automobiles represented by the lightweighting of automobile bodies is an important technical means for energy conservation and consumption reduction. From the perspectives of performance, cost, maintainability, recyclability, and LCA emission evaluation, high-strength steel and ultra-high-strength steel are still more comprehensive and competitive solutions among many materials for lightweighting automobile bodies. Generally, automotive steel with a tensile strength of ≧340 MPa is called high-strength steel, and automotive steel with a tensile strength of ≧780 MPa is called ultra-high-strength steel. In the case of ultra-high-strength steel, since transformation strengthening + precipitation strengthening are the main strengthening means, many alloying elements such as C, Si, Mn, Nb, V, Ti, Cr, Mo, etc. are added to the steel, but not limited to these. In addition to strength, for automotive body materials, especially for materials used in the lower part of the automotive body, corrosion resistance is also emphasized. In addition to the paint coating, automotive steel with a plating layer is superior in corrosion resistance to materials without a plating layer. Common types of plating layers for high-strength steel and ultra-high-strength steel include hot-dip pure zinc plating (GI), hot-dip zinc-iron alloy plating (GA), electro-galvanized pure zinc plating (EG), and zinc-aluminum-magnesium plating (MgAlZn), etc.

[0003] Unlike ordinary steel materials, in plated high-strength steel products, especially in plated ultra-high-strength steel products, due to higher strength and more alloying elements, a phenomenon called Liquid Metal Embrittlement (LME) is likely to occur during the process of resistance spot welding. This principle is mainly because under the action of the stress during welding, the plating layer on the material surface melts due to heat, and the liquid metal infiltrates along the grain boundaries of the steel plate substrate, resulting in a decrease in the bonding force of the grain boundaries and the occurrence of cracks. Factors such as the base metal composition of the welded joint, carbon equivalent, microstructure, composition and weight of the plating layer, pressure, stress, current, heating time, and load holding time during welding all affect the LME phenomenon. However, a large resistivity of the material, high strength of the material, large welding current, large stress of the joint, long welding heating time, and short load holding time all exacerbate the LME phenomenon. In the case of ultra-high-strength steel with a plating layer, a higher carbon equivalent of its base metal, a larger heat input during welding, and higher strength of the base metal itself are also more likely to lead to larger welding stresses. All these adverse factors are significant constraints on the stable batch use of plated ultra-high-strength steel in automobile bodies.

[0004] In the industry, LME cracks in resistance spot welding are generally divided into four types: type A, type B, type C, and type D according to the crack occurrence sites, as shown in Figure 1. Among them, type A cracks occur at the site where the electrode is in direct contact with the material. The temperature at this site is high, and cracks are likely to occur after the welding spatter occurs, having little impact on the performance of the welded joint. However, type B and type C cracks occur on and between the base metals, and type D cracks occur at the shoulder part of the welded joint. All of them affect the performance of the welded joint. In particular, type C cracks are likely to reduce the bonding force of the welded joint and cause the failure of the welded joint, so it is necessary to control their number and length. Regarding this, the following research exists: In the U.S. patent with the registration announcement number US11299793B2 and the title "Steel sheet having excellent resistance to liquid metal embrittlement cracks and method for manufacturing the same", a galvanized steel sheet excellent in resistance to liquid metal embrittlement cracks and a method for manufacturing the same are disclosed. Its composition is in weight percentage: C: 0.04 - 0.35%, Al + Si: 0.99% or less, Mn: 3.5 - 10%, P: 0.05% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.02% or less (excluding 0%), and the balance is Fe and other inevitable impurities. In volume fraction, its microstructure includes 10% or more of retained austenite, 60% or more of annealed martensite, and 20% or less of α - martensite and ε - martensite, and the average thickness of the Mn depletion layer is 0.5 μm or more from the product surface. The invention has a high Mn content and is suitable for production by endless rolling processes such as ESP. In this patent, the crack resistance when applying the resistance spot welding process is not mentioned.

[0005] In the U.S. patent with the registration announcement number US11299793B2 and the title "Steel sheet having excellent resistance to liquid metal embrittlement cracks and method for manufacturing the same", a galvanized steel sheet with excellent resistance to liquid metal embrittlement cracks and a method for manufacturing the same are disclosed. Its composition is in weight percentage: C: 0.04 - 0.35%, Al + Si: 0.99% or less, Mn: 3.5 - 10%, P: 0.05% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.02% or less (excluding 0%), and the balance is Fe and other inevitable impurities. In volume fraction, its microstructure includes 10% or more retained austenite, 60% or more annealed martensite, and 20% or less α - martensite and ε - martensite, and the average thickness of the Mn depletion layer is 0.5 μm or more from the product surface. The invention has a high Mn content and is suitable for production by endless rolling processes such as ESP. In this patent, the crack resistance when applying the resistance spot welding process is not mentioned.

[0006] In view of the above - mentioned drawbacks of the prior art, it is desired to obtain an ultra - high - strength galvanized steel sheet that has better resistance to liquid metal embrittlement (LME) cracks and can suppress the occurrence of Type A, Type B, Type C, and Type D.

Summary of the Invention

Problems to be Solved by the Invention

[0007] An object of the present invention is to provide a 120 - class ultra - high - strength galvanized steel sheet that has excellent resistance to liquid metal embrittlement (LME) cracks and excellent resistance spot weldability. A further object of the present invention is to provide a 120 - class ultra - high - strength galvanized steel sheet with excellent resistance spot weldability. However, in a welded joint combination where at least one layer of the at least two - layer steel sheet is the steel sheet according to the present invention, the welding current ≤ (I splash +I splash*50%) case, when type B cracks and type C cracks do not occur and type A cracks occur, among all type A cracks, type A cracks less than or equal to 1% of the total number occur when the welding current < I splash occurs, and the length of type A cracks is less than or equal to 5% of the base metal plate thickness. When type D cracks occur, among all type D cracks, type D cracks 99.99% or more of the total number occur when the welding current ≧ I splash occurs, and the length of type D cracks is less than or equal to 10% of the base metal plate thickness.

Means for Solving the Problem

[0008] To achieve the above object, the present invention provides a 120-kilogram class ultra-high strength galvanized steel sheet, and the steel sheet C: 0.18 to 0.24%, Mn: 2.3 to 3.0%, Si: 0.5 to 1.7%, Al: 0.02 to 1.0%, 0.55 < Si + Al ≦ 1.75%, C + Si / 30 + Mn / 20 ≦ 0.395%, and at least one of Nb, Ti, B, Cr, Mo, REM, is contained, the balance is Fe and inevitable impurities, provided that taking the thickness of the steel sheet as t, the resistivity R1 of the steel sheet is 0 < R1 ≦ 55 μΩ·cm, and from the interface between the plating layer and the steel sheet matrix to the steel sheet matrix direction, the resistivity R2 of the steel sheet in the range of 0.025t or more to 0.05t or less is 0 < R2 ≦ 15 μΩ·cm, and the resistivity R3 of the material in the range of 0.01t or more to 0.015t or less is 0 < R3 ≦ 35 μΩ·cm, and 1.5R1 1 / 2 -0.1R2 - 0.25R3 > 0 is satisfied.

[0009] Another embodiment of the present invention is a 120-kilogram class ultra-high strength galvanized steel sheet, and in addition to Fe and other inevitable impurities, the steel sheet further C: 0.18 to 0.24%, Mn: 2.3 to 3.0%, Si: 0.5 to 1.7%, Al: 0.02 to 1.0%, 0.55 < Si + Al ≤ 1.75%, C + Si / 30 + Mn / 20 ≤ 0.395%, and at least one of Nb, Ti, B, Cr, Mo, REM, contained, However, Let the thickness of the steel plate be t. The resistivity of the steel plate is 0 < R1 ≤ 55 μΩ·cm. From the interface between the plating layer and the steel plate matrix, in the range of 0.025t or more to 0.05t or less in the steel plate matrix direction, the resistivity of the steel plate is 0 < R2 ≤ 15 μΩ·cm. In the range of 0.01t or more to 0.015t or less, the resistivity of the material is 0 < R3 ≤ 35 μΩ·cm, and 1.5R1 1 / 2 - 0.1R2 - 0.25R3 > 0 is satisfied.

[0010] Furthermore, in the 120 - class ultra - high - strength zinc - plated steel plate provided by the present invention, let the thickness of the steel plate be t. The resistivity R1 of the steel plate is 41 to 55 μΩ·cm. From the interface between the plating layer and the steel plate matrix, in the range of 0.025t or more to 0.05t or less in the steel plate matrix direction, the resistivity R2 of the steel plate is 11 to 15 μΩ·cm. In the range of 0.01t or more to 0.015t or less, the resistivity R3 of the material is 24 to 35 μΩ·cm, and 1.5R1 1 / 2 - 0.1R2 - 0.25R3 > 0 is satisfied.

[0011] In the above - mentioned technical solution of the present invention, the design principle of each chemical element is as follows. C: C is a solid-solution strengthening element commonly used in steel. Due to its high solubility in austenite, in products subjected to the quenching-partitioning process, carbon in martensite localizes in the retained austenite, improving the stability of the retained austenite. By utilizing the TRIP effect of the retained austenite, an improvement in the elongation of the material is achieved. The ultra-high strength galvanized steel sheet according to the present invention has a tensile strength of 1180 MPa or more and an elongation at break of 14% or more. If the C content is too low, the ultra-high strength of the material cannot be ensured, and at the same time, it is disadvantageous for the formation of carbon-rich and stable retained austenite, and it also affects the elongation at break of the material. However, if the C content is too high, the carbon equivalent is significantly increased, which affects the resistance spot weldability, and at the same time, twins are more likely to occur, increasing the crack sensitivity. Therefore, in the present invention, the mass percentage of C is controlled to be 0.18 - 0.24%.

[0012] Mn: Mn can improve the hardenability of steel, lower the critical transformation temperature, and improve the strength of steel. At the same time, Mn significantly expands the austenite phase region, A c3 , M s , and M f points are lowered, the stability of austenite is improved, and it can also contribute to the improvement of the elongation of steel. However, if the Mn content is too high, the carbon equivalent is significantly increased, which is disadvantageous for the resistance spot weldability. At the same time, it severely enhances the tendency of grain coarsening, reduces the plasticity and toughness of steel, and deteriorates the corrosion resistance. Therefore, in the present invention, the mass percentage of Mn is controlled to be 2.3 - 3.0%.

[0013] Si: Si has an extremely low solubility in carbides, strongly suppresses the precipitation of cementite, promotes the localization of carbon in the retained austenite, improves the stability of the retained austenite, and thereby can improve the strength and formability of the material. However, if the Si content is too high, Si-containing oxides are likely to form in the annealing process, so the plating property of the steel sheet surface deteriorates, which is disadvantageous for obtaining a plating layer of good quality. However, if the Si content is too high, the carbon equivalent also increases, which is disadvantageous for the resistance spot weldability. Therefore, in the present invention, the mass percentage of Si is controlled to be 0.5 - 1.7%.

[0014] Al: Al has an effect similar to Si in suppressing cementite and improving the stability of retained austenite. At the same time, it can not only improve the mechanical stability of retained austenite, but also improve the thermal stability of retained austenite. Furthermore, Al can form insoluble particles that are finely and dispersedly distributed with C and N to refine crystal grains, so Al can partially replace Si in steel. However, the strengthening effect of Al is weaker than that of Si. When the Al content is too high, the A c3 temperature of the steel rises significantly, increasing the difficulty of continuous casting and the risk of billet cracking. Therefore, in the present invention, the mass percentage of Al is controlled to be 0.02 - 1.0%.

[0015] In the design of the chemical composition of the present invention, it is further necessary to control the mass percentage content of Al and Si elements so that 0.55% < Al + Si ≤ 1.75% is satisfied. The reason for controlling this technical feature is that an appropriate amount of Al and Si can ensure a certain strengthening effect, promote the stabilization of retained austenite, and ensure the strength and elongation rate of the material. Moreover, there are no production obstacles such as the difficulty of continuous casting, billet cracking, and deterioration of plating properties.

[0016] The resistivity is closely related to any of alloying elements, crystal structure, crystal defects, and solid solution effects. The gradient change of the resistivity in the thickness direction reflects the change of the microstructure. When the resistivity decreases from the surface layer in a certain thickness direction, the total heat input during the welding process can be reduced, and the occurrence of cracks can be advantageously prevented. Therefore, in the ultra-high strength galvanized steel sheet according to the present invention, taking the thickness of the steel sheet as t, the resistivity R1 of the steel sheet is 0 < R1 ≤ 55 μΩ·cm, and the resistivity R2 of the steel sheet in the range of 0.025t or more to 0.05t or less from the interface between the plating layer and the steel sheet matrix in the direction of the steel sheet matrix is 0 < R2 ≤ 15 μΩ·cm, and the resistivity R3 of the steel sheet in the range of 0.01t or more to 0.015t or less is 0 < R3 ≤ 35 μΩ·cm, and 1.5R1 1 / 2-0.1R2 - 0.25R3 > 0 is satisfied. However, the ranges of R1, R2, and R3 and their relationships represent the average resistivity level of the material and the resistivity level in the region where LME cracks can occur during the resistance spot welding process. In particular, 0 < R2 ≤ 15 μΩ·cm, 0 < R3 ≤ 35 μΩ·cm, and 1.5R1 1 / 2 When -0.1R2 - 0.25R3 > 0, the following can be ensured: welding current ≤ (I splash +I splash *50%). When type B cracks and type C cracks do not occur and type A cracks occur, among all type A cracks, type A cracks accounting for 1% or less of the total number occur when the welding current < I splash and the length of type A cracks is 5% or less of the base metal plate thickness. When type D cracks occur, among all type D cracks, type D cracks accounting for 99.99% or more of the total number occur when the welding current ≥ I splash and the length of type D cracks is 10% or less of the base metal plate thickness. However, I splash is the minimum current when sputtering occurs.

[0017] Furthermore, in the 120-kilogram class ultra-high-strength galvanized steel sheet according to the present invention, the contents of Nb, Ti, B, Cr, Mo, and REM are: 0 ≤ Nb ≤ 0.1%; 0 ≤ Ti ≤ 0.1%; 0 ≤ B ≤ 0.003%; 0 ≤ Cr ≤ 0.1%; 0 ≤ Mo ≤ 0.1%; 0 ≤ REM ≤ 0.05%.

[0018] Furthermore, in the 120-kilogram class ultra-high-strength galvanized steel sheet according to the present invention, when contained, the contents of Nb, Ti, B, Cr, Mo, and REM are: Nb: 0.08 - 0.1%; Ti: 0.01 - 0.02%; B: 0.0004 - 0.0023%; Cr: 0.05 - 0.1%; Mo: 0.02 - 0.1%; REM: 0.0035 - 0.05%.

[0019] Nb and Ti are carbide - forming elements, which can suppress the precipitation of cementite. Fine Nb and Ti carbides also have the effect of refining crystal grains and improving strength. However, Nb and Ti carbides are disadvantageous to the stabilization of retained austenite, and when a large amount of Nb and Ti are added, the cost of the material alloy will increase. Therefore, in the present invention, the mass percentage of Nb and Ti is controlled to be 0 - 0.1%.

[0020] B tends to be unevenly distributed at the austenite grain boundaries. By suppressing austenite transformation and improving the hardenability of steel, it can improve strength. However, if the B content is too high, the formability of the steel will deteriorate and the risk of cracking will increase. Therefore, in the present invention, the mass percentage of B is controlled to be 0 - 0.003%.

[0021] Both Cr and Mo are elements that promote transformation strengthening and enhance the stability of austenite, and can improve the resistance of austenite to tempering decomposition. However, if the contents of Cr and Mo are too high, it will inhibit bainite transformation and is disadvantageous to the localization of C in austenite, thus weakening the stability of austenite. At the same time, Mo also significantly improves the resistance of steel to cold rolling deformation and increases the difficulty of cold rolling production. Therefore, in the present invention, the mass percentages of both Cr and Mo are controlled to be 0 - 0.1%.

[0022] REM is the general term for rare - earth elements. The rare - earths commonly used in steel are mainly mixtures of La and Ce, but the inclusion of other rare - earth elements other than La and Ce is not excluded. REM has the effect of purifying the grain boundaries and transforming inclusions. However, if its content is too high, the formability of the material will be impaired. Therefore, in the present invention, the mass percentage of REM is controlled to be 0.01% or less.

[0023] Furthermore, among other inevitable impurities in the present invention, P ≤ 0.015%, S ≤ 0.010%, and N ≤ 0.008%.

[0024] P, S, and N are all inevitable impurity elements in steel. If the P content is too high, the grain boundaries will be weakened, the risk of brittle fracture will increase, and the weldability will decrease. As an impurity element, S affects the formability and weldability of steel. N can improve the stability of austenite and has a certain strengthening effect. However, if the N content is too high, the risk of brittle fracture will increase, and at the same time, the precipitation of AlN is likely to occur, reducing the quality of continuous casting. Therefore, in the present invention, the mass percentages of P, S, and N are controlled to be 0.015% or less, 0.010% or less, and 0.010% or less, respectively.

[0025] Furthermore, in the 120-kilogram-class ultra-high-strength galvanized steel sheet according to the present invention, when a type III sample compliant with the ISO 6892-1 standard perpendicular to the rolling direction is tensile-tested at room temperature, the tensile strength of the steel sheet is ≧1180 MPa, the yield strength is ≧800 MPa, the elongation at break is ≧14%, and the hole expansion ratio is ≧30%.

[0026] Furthermore, in the 120-kilogram-class ultra-high-strength galvanized steel sheet according to the present invention, a GA sheet or a GI sheet is arranged as the steel sheet.

[0027] Furthermore, the thickness of the 120-kilogram-class ultra-high-strength galvanized steel sheet according to the present invention is 0.8 - 2.5 mm.

[0028] Furthermore, in the 120-kilogram-class ultra-high-strength galvanized steel sheet according to the present invention, when the welding current ≦1.5*I splash no type B and type C LME cracks occur. When type D cracks occur, their length is less than 10% of the base metal plate thickness. When the welding current <I splash no type B cracks, type C cracks, and type D cracks occur. When type A cracks occur, their length is less than 5% of the base metal plate thickness; provided that I splash is the minimum current when sputtering occurs.

[0029] The second aspect of the present invention is a method for manufacturing the above-mentioned 120-kilogram-class ultra-high-strength galvanized steel sheet, characterized by including the following steps.

[0030] (1) Smelting and continuous casting; (2) Hot rolling; (3) Pickling and cold rolling; (4) Continuous annealing; (5) Zinc plating.

[0031] In the step (1), a billet whose chemical composition meets the requirements of the present invention is obtained. In the step (2), the billets in the step (1) are heated in sequence, finish rolled, laminar cooled, and coiled to obtain a hot-rolled coil. In the step (2), the heating temperature is in the range of 1150 - 1300°C, the finish rolling temperature is A c3 ~1000°C, the holding temperature for laminar cooling is in the range of (A c1 ±45°C), and the residence time of laminar cooling is 5 - 30 s. Further, it is cooled to 550 - 650°C and coiled, and the coiling temperature is T C and the coiled steel coil is held for 30 - 300 min in the range of (coiling temperature T C ±30°C).

[0032] Furthermore, in the step (2), the heating temperature is 1165 - 1270°C; the finish rolling temperature is 885 - 945°C; the holding temperature for laminar cooling is 680 - 720°C, and the residence time is 7 - 26 s; the coiling temperature is 550 - 645°C, and the holding time after coiling is 45 - 270 min.

[0033] In the step (3), the hot-rolled coil obtained in the step (2) is pickled and cold-rolled to obtain a full-hard coil.

[0034] In the step (4), the full-hard coil obtained in the step (3) is subjected to multi-stage annealing treatment, and the multi-stage annealing treatment includes the following (a) - (d): (a) In the first-stage annealing, the full-hard coil is heated to a first-stage temperature range of 600°C or higher to (A c1 +40°C) to obtain a steel coil; (b) In the second-stage annealing, the steel coil obtained in (a) is continuously (Ac1 (A) to (+50°C)~ c3 (A) to (+80°C) or c1 Heat to the second-stage temperature range of (A) to (+50°C)~900°C, hold for 30~300 s to obtain a strip steel. The upper limit of the second-stage temperature range is the smaller of (A c3 (+80°C) and 900°C; (c) In the third-stage annealing, heat the strip steel obtained in (b) at a cooling rate V of not less than a predetermined value 2-3 to the third-stage temperature range M s ~M f heat it and hold for 10~120 s; (d) In the fourth-stage annealing, heat the strip steel obtained in (c) again to the fourth-stage temperature range of (350°C~T ZP ) and hold for 15~90 s.

[0035] In step (4), the atmosphere in (a) contains O2 with a volume content of 0.01~0.5%, and the balance is N2 and unavoidable impurities; the atmosphere in (b) contains H2 with a volume content of at least 1.5% and water vapor with a volume content of 0.2%, and the balance is N2 and unavoidable impurities, and the dew point is -25~10°C; V 2-3 in (c) represents the cooling rate and is 50°C / s or more, for example, 50~1000°C / s; T ZP in (d) is the zinc pot temperature; preferably, T ZP is 458~461°C.

[0036] Furthermore, in step (4), the heating temperature in (a) is 680~720°C; the heating temperature in (b) is 830~900°C, and the soaking time is 30~145 s; the third-stage temperature in (c) is 220~310°C, and the holding time is 25~110 s; the heating temperature in (d) is 355~420°C, and the holding time is 20~86 s.

[0037] In the method for manufacturing an ultra-high-strength galvanized steel sheet according to the present invention, in the above step (2), the hot-rolled strip steel is (A c1Cooling to a range of ±45°C and holding for 5 to 30 s is to control the laminar flow cooling process to perform stepwise cooling, thereby causing as much ferrite transformation as possible in the hot-rolled strip within the residence time, reducing the variation in microstructure and properties in the strip width direction, and improving the strip shape quality of the strip. After the coiled steel coil is (T C Holding for 30 to 300 min in the range of ±30°C) is to provide sufficient time for the bainite transformation or pearlite transformation of the strip, reduce the formation of the hard phase martensite, and further lower the strength of the strip, so as to facilitate cold rolling.

[0038] In the method for manufacturing an ultra-high-strength galvanized steel sheet according to the present invention, the multi-stage annealing in the step (4) is an austempering process. The first-stage annealing is a process of preheating and pre-oxidizing the strip. By controlling the O2 content in the atmosphere, external oxidation of elements such as Si and Mn in the steel can be suppressed, and oxidation of Fe can be promoted. The second-stage annealing is a heating / soaking process and also an internal oxidation process. By controlling the annealing temperature, the strip is heated in the austenite single-phase region or the ferrite + austenite two-phase region to obtain an appropriate ratio of austenite, and by controlling the annealing atmosphere and dew point, elements such as Si and Mn are internally oxidized. The third-stage annealing is a quenching process. The strip is quenched to M s ~M f by quenching at a cooling rate higher than the cooling rate in the high-hydrogen cooling mode, so that the austenite generated in the heating / soaking stage is transformed into martensite + retained austenite, and the amount of the generated martensite is determined by the height of the quenching temperature. The fourth-stage annealing is a process of reheating and partitioning. By controlling the annealing temperature to (350°C to T ZP ), it can not only promote the diffusion and localization of carbon elements in martensite into austenite, but also avoid a significant decrease in strength caused by severe martensite tempering at too high partitioning temperatures. After the fourth-stage annealing, the strip is (T ZPPut it into a zinc pot at a temperature of ±15°C to complete zinc plating; in the case of a molten pure zinc plating layer, after taking the strip out of the zinc pot, cool it to room temperature to obtain the final product; in the case of a molten zinc-iron alloy plating layer, after taking the strip out of the zinc pot, reheat it for alloying; if the alloying temperature is too low, insufficient alloying will occur, but if the alloying temperature is too high, the stability of retained austenite will decrease and decompose, affecting the elongation rate of the final product. Therefore, the alloying temperature is controlled at (T ZP -20°C) to (T ZP +35°C); if the alloying time is too short, sufficient alloying of the strip cannot be achieved, but if the alloying time is too long, the iron content in the plating layer will be too high, deteriorating the choking resistance of the plating layer. Therefore, the alloying time is controlled at 5 - 30 s.

[0039] To obtain better implementation effects, in some preferred embodiments, to prevent the heating temperature from being too low and the finishing rolling force from being too large, or the heating temperature from being too high and the slab from being overburned with crystal grains becoming too coarse, the billet heating temperature in step (2) is controlled at 1200 - 1280°C.

[0040] To obtain better implementation effects, in some preferred embodiments, in step (2), the finishing rolling temperature is controlled at (A c3 +20°C) to 950°C.

[0041] To obtain better implementation effects, in some preferred embodiments, during the laminar flow cooling process of the strip in step (2), the temperature is controlled in the range of (A c1 -20°C) to (A c1 +30°C) and kept warm for 7 - 16 s.

[0042] To obtain better implementation effects, in some preferred embodiments, during the heat preservation process after coiling the strip in step (2), the residence time is controlled at 120 - 240 min.

[0043] In order to obtain better implementation effects, in some preferred embodiments, in the annealing process in step (4), the annealing temperature in the second stage is (A c1 +70 °C) to (A c3 +80 °C) or (A c1 +70 °C) to 900 °C, the heat preservation time is 35 to 120 s, and the upper limit of the temperature range in the second stage is the smaller one of (A c3 +80 °C) and 900 °C.

[0044] In order to obtain better implementation effects, in some preferred embodiments, in the annealing process in step (4), the atmosphere in the temperature range of the second stage contains H2 with a volume content of 4 to 25%, and contains water vapor with a volume content of 0.05% to 0.10%, the balance being N2 and inevitable impurities, and the dew point is -15 to 0 °C.

[0045] In order to obtain better implementation effects, in some preferred embodiments, in the annealing process in step (4), the temperature in the fourth stage is 350 °C to (T ZP -35 °C), and the heat preservation time is 30 to 60 s.

[0046] In order to obtain better implementation effects, in some preferred embodiments, in the annealing process in step (4), the strip steel is put into the zinc pot at a temperature of (T ZP ±10 °C).

[0047] In order to obtain better implementation effects, in some preferred embodiments, in the annealing process in step (4), the alloying temperature is (T ZP -10 °C) to (T ZP +25 °C), and the heat preservation time is 10 to 20 s.

[0048] The 120-kilogram-class ultra-high-strength galvanized steel sheet with excellent resistance spot weldability and its manufacturing method according to the present invention have the following advantages and beneficial effects compared with the prior art: (1) The present invention first achieved the suppression of LME cracking during the resistance spot welding process by limiting the resistivity in different regions in the thickness direction of the steel sheet and the average resistivity of the steel sheet for a 120-kilogram class ultra-high-strength galvanized steel sheet, and an ultra-high-strength galvanized steel sheet with excellent resistance spot weldability was obtained.

[0049] (2) In a welded joint combination where at least one layer of the at least two-layer steel sheet is the 120-kilogram class ultra-high-strength galvanized steel sheet according to the present invention, when the welding current ≤ (I splash + I splash* × 50%), type B cracks and type C cracks do not occur. When type A cracks occur, among all type A cracks, type A cracks accounting for 1% or less of the total number occur when the welding current < I splash , and the length of type A cracks is 5% or less of the base metal plate thickness. When type D cracks occur, among all type D cracks, type D cracks accounting for 99.99% or more of the total number occur when the welding current ≥ I splash , and the length of type D cracks is 10% or less of the base metal plate thickness. However, I splash is the minimum current when sputtering occurs.

[0050] (3) The ultra-high-strength galvanized steel sheet according to the present invention has a simple composition. Through precise control of the processes in laminar flow cooling in hot rolling, coiling and heat preservation in hot rolling, and continuous annealing process, the obtained steel sheet has a tensile strength ≥ 1180 MPa, a yield strength ≥ 800 MPa, an elongation at break ≥ 14%, an expansion rate ≥ 30%, and excellent resistance spot weldability.

[0051] (4) The ultra-high-strength galvanized steel sheet according to the present invention has simple process equipment required for production, enables stable batch production, can also be used for manufacturing body parts with complex shapes due to the comprehensive performance of the material, and has excellent resistance spot weldability and corrosion resistance.

Brief Description of the Drawings

[0052]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0053] Hereinafter, based on specific examples, the specific embodiments of the present invention will be further interpreted and described, but such interpretation and description do not limit the technical solution of the present invention.

[0054] In the present invention, zinc plating refers to hot-dip zinc plating or hot-dip zinc-iron alloy plating.

[0055] In the present invention, A c1 refers to the transformation temperature from pearlite to austenite during heating, and the unit is °C.

[0056] In the present invention, A c3 refers to the end temperature of the transformation to austenite during heating, and the unit is °C.

[0057] In the present invention, T C refers to the coiling temperature in step (2), and the unit is °C. In the present invention, M s is the temperature at which "martensite" appears, and M f is the temperature at which complete "martensite" transformation occurs, and the unit is °C.

[0058] In the present invention, T ZP represents the zinc pot temperature, and the unit is °C. In the present invention, V 2-3 represents the cooling rate.

Examples

[0059] Examples 1 to 12 and Comparative Examples 1 to 2 The mass percentages of each chemical element in the ultra-high strength galvanized steel sheets according to Examples 1 to 12 and the galvanized steel sheets according to Comparative Examples 1 to 2 are shown in Table 1.

[0060] [Table 1]

[0061] The ultra-high strength galvanized steel sheets according to Examples 1 to 12 of the present invention were manufactured by the following steps: (1) Steelmaking and continuous casting: A billet having a chemical composition satisfying the steel sheet composition of the present invention was obtained; (2) Hot rolling: The billet obtained in step (1) was heated at 1150 to 1300 °C, and finish-rolled in the range of A c3 ~1000 °C, and the finish-rolled strip steel was cooled in the range of (A c1 ±45 °C), and the residence time of laminar flow cooling was 5 to 30 s, and further cooled to 550 to 650 °C and coiled, and the coiling temperature was T C , and the coiled steel coil was retained in the range of (T C ±30 °C) for 30 to 300 min to obtain a hot-rolled coil; (3) Pickling and cold rolling: The hot-rolled coil obtained in step (2) was pickled and cold-rolled to obtain a full-hard coil; (4) Continuous annealing: The full-hard coil obtained in step (3) was subjected to multi-stage heat treatment; (a) In the first-stage annealing, the full-hard coil was heated to the first-stage temperature range of 600 °C or higher to (A c1 +40 °C) to obtain a steel coil; (b) In the second-stage annealing, the steel coil obtained in (a) was continuously heated to the second-stage temperature range of (A c1 +50 °C) to (A c3 +80 °C) or (A c1 +50 °C) to 900 °C, and held for 30 to 300 s to obtain a strip steel; (c) In the third-stage annealing, the strip steel obtained in (b) was heated to the third-stage temperature range M 2-3 at a cooling rate V s of a predetermined value or higher to M f , and held for 10 to 120 s; (d) In the annealing of the fourth stage, the strip steel obtained in (c) is heated again to the temperature range of the fourth stage of 350°C to T ZP and held for 15 to 90 s; However, in step (4), the atmosphere in (a) contains O2 with a volume content of 0.01 to 0.5%, and the balance is N2 and inevitable impurities; the atmosphere in (b) contains H2 with a volume content of at least 1.5% and water vapor with a volume content of 0.2%, and the balance is N2 and inevitable impurities, the dew point is -20 to 15°C, and the upper limit of the temperature range of the second stage is the smaller of A c3 + 80°C and 900°C; V in (c) 2-3 represents the cooling rate and is 50°C / s or more; T in (d) ZP is the zinc pot temperature; (5) Zinc plating: The strip steel obtained by annealing the fourth stage of (d) in step (4) is put into a zinc pot at a temperature of (T ZP ± 15°C) to complete zinc plating, and a zinc-plated steel sheet is obtained; however, when the zinc-plated steel sheet is a hot-dip galvanized layer steel sheet, after the steel sheet having a galvanized layer is taken out of the zinc pot, it is cooled to room temperature; when the zinc-plated steel sheet is a hot-dip zinc-iron alloy galvanized layer steel sheet, after the steel sheet having a hot-dip zinc-iron alloy galvanized layer is taken out of the zinc pot, it is alloyed by holding for 5 s to 60 s in the range of (T ZP - 20°C) to (T ZP + 35°C) and then cooled to room temperature.

[0062] Specific process parameters of the ultra-high strength zinc-plated steel sheets according to Examples 1 to 12 are shown in Tables 2-1 and 2-2.

[0063]

Table 2-1

[0064]

Table 2-2

[0065] GI: The plating layer is a pure zinc layer, GA: The plating layer is a zinc-iron alloy.

[0066] Regarding the ultra-high strength zinc-plated steel sheets according to Examples 1 to 12 and the zinc-plated steel sheets according to Comparative Examples 1 to 2, the mechanical properties and resistance spot weldability were measured, and the obtained measurement results are shown in Table 3.

[0067] However, the measurement methods for the mechanical properties, resistivity, and resistance spot weldability were as follows: Mechanical properties: In the direction perpendicular to the rolling direction of the steel sheet, tensile samples were processed according to the requirements of Type III samples in ISO 6892-1 standard, and tensile tests were carried out at room temperature to measure the tensile strength TS, yield strength YS, and elongation at break EL. It should be emphasized that due to the differences in measurement methods and sample geometric dimensions, there are differences between the elongation at break measured according to ISO 6892-1 standard and the elongation at break measured according to JIS Z2241 standard or GB / T 228.1 standard. The differences in the measured values of the elongation at break due to the differences in reference standards all fall within the protection scope of the ultra-high strength zinc-plated steel sheet according to the present invention.

[0068] Hole expansion rate: Samples were processed according to ISO 16630 standard, and the hole expansion rate HER was measured at room temperature according to the requirements of the standard. It should be emphasized that due to the differences in measurement methods and sample geometric dimensions, there are differences between the hole expansion rate measured according to ISO 16630 standard and the hole expansion rate measured according to JFS T1001 standard or GB / T 15825.4 standard. The differences in the measured values of the hole expansion rate due to the differences in reference standards all fall within the protection scope of the ultra-high strength zinc-plated steel sheet according to the present invention.

[0069] Resistivity: Samples of the steel sheet with a predetermined area were taken, and samples with a thickness of the entire thickness t of the steel sheet, 0.025t or more and 0.05t or less, and 0.01t or more and 0.015t or less were taken using processing means including but not limited to a grinding machine, a wire cutter, and a milling machine. The resistivity R1, R2, and R3 of the samples were measured with a resistivity measuring device.

[0070] LME cracks: After cutting, polishing, and corroding the welded joint, the cross-section of the joint was observed under a microscope (usually an optical microscope) at a predetermined magnification, and the lengths of various types of cracks in the observed welded joint were determined.

[0071] Regarding the ultra-high-strength zinc-plated steel sheets according to Examples 1 to 12, the mechanical properties, resistivity, and resistance spot weldability were measured, and the obtained measurement results are shown in Table 3, provided that I splash is the minimum current when sputtering occurs. The measurement of the resistance spot weldability of the zinc-plated steel sheets according to Comparative Examples 1 to 2 is shown in Table 4.

[0072] [Table 3]

[0073] TS: Tensile strength; YS: Yield strength; TEL: Elongation at break; HER: Hole expansion rate; N / D: Not detected.

[0074] Regarding the zinc-plated steel sheets according to Comparative Examples 1 to 2, the resistance spot weldability was measured, and the obtained measurement results are shown in Table 4.

[0075] [Table 4]

[0076] As can be seen from Tables 1 to 3, since the chemical elements of the steel types used in Examples 1 to 12 of the present invention meet the requirements of the present invention, the ultra-high-strength zinc-plated steel sheets obtained by the method according to the present invention in Examples 1 to 12 have a tensile strength of ≧1180 MPa, a yield strength of ≧800 MPa, an elongation at break of ≧14%, a hole expansion rate of ≧30%, and a resistivity that meets the requirements of the present invention. Therefore, the steel sheets according to Examples 1 to 12 have a welding current of ≦ (I splash +I splash*50%), in all cases, LME cracks of type B and type C do not occur, and the welding current <I splash In the case of splash , LME cracks of type A do not appear, or when LME cracks of type A appear, the length of the type A cracks is 5% or less of the base metal plate thickness. For example, in Examples 2 and 3, the lengths of the LME cracks of type A were 0.5% of the base metal plate thickness, respectively. Welding current ≧ I splash In the case of splash , LME cracks of type D do not appear, or when LME cracks of type D appear, the lengths of the type D cracks are all 10% or less of the base metal plate thickness. For example, in Example 6 where the length of the LME crack of type D is the longest among the examples, the length of the type D crack is 9.2% of the base metal plate thickness.

[0077] On the other hand, the steel plates according to Comparative Examples 1 and 2 are conventional steel plates and are not manufactured by the manufacturing method of the steel plate according to the present invention. In the chemical elements of the steel types of the steel plates according to Comparative Example 1, Al and Sn do not satisfy the requirements of the present invention, and in the chemical elements of the steel types according to Comparative Example 2, Mn, Al, Cr, and Sb do not satisfy the requirements of the present invention. Therefore, when resistance spot welding is performed, in the steel plates according to Comparative Examples 1 and 2, type C cracks with a large influence on the base metal occur, and the generated type C cracks are as large as 32.3% and 39.1% of the base metal plate thickness, respectively. Compared with the steel materials according to the examples of the present invention, the steel materials according to the comparative examples are clearly inferior in resistance to liquid metal embrittlement LME cracks and resistance spot weldability.

[0078] As can be seen from the above, in the present invention, by optimizing the chemical elements and controlling the manufacturing process, the LME cracks of the ultra-high strength galvanized steel plates according to Examples 1 to 12 obtained are suppressed, and the resistance spot weldability is excellent.

[0079] In summary, the present disclosure can obtain an ultra-high strength galvanized steel having excellent tensile strength, yield strength, elongation at break, hole expansion rate, and excellent resistance spot weldability by combining a reasonable chemical composition design and an optimized process. In that case, in a welded joint combination in which at least one layer of the at least two layers of steel plates is a steel plate according to the present invention, the welding current ≦ (Isplash +I splash When (*50%), no type B cracks or type C cracks occur. When type A cracks occur, type A cracks of less than 1% of the total number occur at welding current <I splash occur in the case of, and the length of type A cracks is 5% or less of the base metal plate thickness. When type D cracks occur, 99.99% or more of the total number of type D cracks occur at welding current ≧I splash occur in the case of, and the length of type D cracks is 10% or less of the base metal plate thickness. The 120-kilogram-class ultra-high-strength galvanized steel sheet according to the present disclosure has excellent liquid metal embrittlement LME crack resistance and resistance spot weldability, enables stable batch production, and can also handle the manufacture of complex-shaped body parts due to the comprehensive performance of the material. Moreover, it is excellent in resistance spot weldability and corrosion resistance, can be effectively applied to the manufacture of automobile body structures, meets the current development needs of automobile steels regarding vehicle weight reduction and safety, and has good application prospects.

[0080] The prior art part within the protection scope of the present invention is not limited to the embodiments described in the application documents of this application. It should be explained that prior art (including, but not limited to, prior patent documents, prior published publications, prior public uses, etc.) that does not conflict with the solution of the present invention is all incorporated into the protection scope of the present invention. Also, the combination of each technical feature in this application is not limited to the combination described in the claims of this application or the combination described in the specific embodiments. As long as they do not conflict with each other, all the technical features described in this application can be freely combined or joined in any form.

[0081] Furthermore, it should also be noted that the above-mentioned embodiments are only specific embodiments of the present invention. The present invention is not limited to the above embodiments, and it is obvious that any similar changes or modifications that those skilled in the art can directly derive from or easily conceive from the disclosure content of the present invention are all included in the protection scope of the present invention.

Claims

1. A 120-kilogram class ultra-high strength galvanized steel sheet, wherein the steel sheet C: 0.18 - 0.24%, Mn: 2.3 - 3.0%, Si: 0.5 - 1.7%, Al: 0.02 - 1.0%, 0.55 < Si + Al ≤ 1.75%, C + Si / 30 + Mn / 20 ≤ 0.395%, and at least one of Nb, Ti, B, Cr, Mo, REM, is contained, the balance being Fe and unavoidable impurities, provided that Let the thickness of the steel plate be \(t\), and the resistivity \(R\) of the steel plate １ is such that \(0 \lt R\) １ \(\leq 55\ \mu\Omega\cdot cm\), and the resistivity \(R\) of the steel plate in the range of \(0.025t\) or more to \(0.05t\) or less in the direction of the steel plate matrix from the interface between the plating layer and the steel plate matrix ２ is such that \(0 \lt R\) ２ \(\leq 15\ \mu\Omega\cdot cm\), and the resistivity \(R\) of the material in the range of \(0.01t\) or more to \(0.015t\) or less ３ is such that \(0 \lt R\) ３ \(\leq 35\ \mu\Omega\cdot cm\), and \(1.5R\) １ １／２ \(- 0.1R\) ２ \(- 0.25R\) ３ satisfies \(> 0\) characterized in that, a 120-kilogram class ultra-high strength galvanized steel sheet.

2. A 120-kilogram class ultra-high strength galvanized steel sheet, wherein the steel sheet, in addition to Fe and other unavoidable impurities, further C: 0.18 - 0.24%, Mn: 2.3 - 3.0%, Si: 0.5 - 1.7%, Al: 0.02 - 1.0%, 0.55 < Si + Al ≤ 1.75%, C + Si / 30 + Mn / 20 ≤ 0.395%, and at least one of Nb, Ti, B, Cr, Mo, REM, is contained, provided that Let the thickness of the steel plate be t, and the resistivity R of the steel plate １ is 0 < R １ ≤ 55 μΩ·cm, and the resistivity R of the steel plate in the range of 0.025t or more to 0.05t or less in the steel plate matrix direction from the interface between the plating layer and the steel plate matrix ２ is 0 < R ２ ≤ 15 μΩ·cm, and the resistivity R of the material in the range of 0.01t or more to 0.015t or less ３ is 0 < R ３ ≤ 35 μΩ·cm, and 1.5R １ １／２ − 0.1R ２ − 0.25R ３ satisfies > 0 characterized in that, a 120-kilogram class ultra-high strength galvanized steel sheet.

3. The content of the Nb, Ti, B, Cr, Mo, REM is: 0 ≤ Nb ≤ 0.1%; 0 ≤ Ti ≤ 0.1%; 0 ≤ B ≤ 0.003%; 0 ≤ Cr ≤ 0.1%; 0 ≤ Mo ≤ 0.1%; 0 ≤ REM ≤ 0.05% characterized in that, the 120-kilogram class ultra-high strength galvanized steel sheet according to Claim 1 or 2.

4. When contained, the content of Nb, Ti, B, Cr, Mo, REM is: Nb: 0.08 - 0.1%; Ti: 0.01 - 0.02%; B: 0.0004 - 0.0023%; Cr: 0.05 - 0.1%; Mo: 0.02 - 0.1%; REM: 0.0035 - 0.05% characterized in that, the 120-kilogram class ultra-high strength galvanized steel sheet according to Claim 3.

5. Among other unavoidable impurities, P ≤ 0.015%, S ≤ 0.010%, N ≤ 0.010% characterized in that, the 120-kilogram class ultra-high strength galvanized steel sheet according to Claim 1 or 2.

6. When a type III sample conforming to ISO 6892-1 standard perpendicular to the rolling direction is tensile tested at room temperature, the tensile strength of the steel sheet is ≥ 1180 MPa, the yield strength is ≥ 800 MPa, the elongation at break is ≥ 14%, and the hole expansion ratio is ≥ 30%, characterized in that, the 120-kilogram class ultra-high strength galvanized steel sheet according to Claim 1 or 2.

7. Let the thickness of the steel sheet be t, and the resistivity R of the steel sheet １ is 41 to 55 μΩ·cm, and the resistivity R of the steel sheet in the range of 0.025t or more and 0.05t or less in the steel sheet matrix direction from the interface between the plating layer and the steel sheet matrix ２ is 11 to 15 μΩ·cm, and the resistivity R of the material in the range of 0.01t or more and 0.015t or less ３ is 24 to 35 μΩ·cm, and 1.5R １ １／２ −0.1R ２ −0.25R ３ > 0 is satisfied, and the 120-kilogram-class ultra-high-strength zinc-plated steel sheet according to claim 1 or 2.

8. Welding current ≤ 1.5 * I ｓｐｌａｓｈ In the case where type B and type C LME cracks do not occur, and when type D cracks occur, the length thereof is less than 10% of the base material plate thickness, and welding current < I ｓｐｌａｓｈ In the case where type B cracks, type C cracks, and type D cracks do not occur, and when type A cracks occur, the length thereof is less than 5% of the base material plate thickness; provided that I ｓｐｌａｓｈ is the minimum current when sputtering occurs, and the 120-kilogram class ultra-high-strength galvanized steel sheet according to claim 1

9. A method for manufacturing a 120-kilogram-class ultra-high-strength galvanized steel sheet according to any one of claims 1 to 8, characterized by including the following steps. (1) Performing refining and continuous casting to obtain a billet that satisfies the composition of the steel sheet according to any one of claims 1 to 5; (2) Hot rolling: Heating the billet in step (1), finish rolling, laminar cooling, and coiling to obtain a hot-rolled coil; (3) Pickling and cold rolling: Pickling the hot-rolled coil obtained in step (2) and cold rolling it to obtain a full-hard coil; (4) Continuous annealing: Subjecting the full-hard coil obtained in step (3) to multi-stage heat treatment to obtain a strip steel; (5) Zinc plating: The strip steel obtained in step (4) is placed in a zinc pot at a temperature of (zinc pot temperature T ＺＰ ± 15 °C) to complete zinc plating and obtain a zinc-plated steel sheet.

10. A method for manufacturing a 120-kilogram-class ultra-high-strength galvanized steel sheet according to claim 9, characterized in that the multi-stage annealing treatment includes the following (a) to (d). In the first annealing step, heat the full-hard coil to a first-step temperature range of 600 °C or higher to (A ｃ１ + 40 °C) to obtain a steel coil; In the second annealing step, the steel coil obtained in (a) is subsequently heated to a second temperature range of (A ｃ１ + 50 °C) to (A ｃ３ + 80 °C) or (A ｃ１ + 50 °C) to 900 °C, held for 30 to 300 s to obtain a strip steel, and the upper limit of the second temperature range is taken as the smaller of (A ｃ３ + 80 °C) and 900 °C; In the annealing of the third stage, the strip steel obtained in (b) is heated at a cooling rate V equal to or higher than a predetermined value ２－３ to the third-stage temperature range M ｓ ~M ｆ and held for 10 to 120 s; In the annealing of the fourth stage, the strip steel obtained in (c) was heated again to the temperature range of the fourth stage from 350°C to T ＺＰ and held for 15 to 90 s; However, A ｃ１ is the transformation temperature from pearlite to austenite during heating, and A ｃ３ is the end temperature of transformation to austenite during heating. M ｓ is the temperature at which martensite begins to appear, and M ｆ is the temperature at which it completely transforms into martensite. V ２－３ represents the cooling rate and is 50 °C / s or more.

11. The heating temperature in the above (a) is 680 to 720 °C; The heating temperature in the above (b) is 830 to 900 °C, and the soaking time is 30 to 145 s; The temperature of the third stage in the above (c) is 220 to 310 °C, and the heat preservation time is 25 to 110 s; The heating temperature in the above (d) is 355 to 420 °C, and the heat preservation time is 20 to 86 s A method for manufacturing a 120-kilogram-class ultra-high-strength galvanized steel sheet according to claim 10, characterized by the above.

12. In step (4), the atmosphere in (a) contains O with a volume content of 0.01 to 0.5%, and the balance is N ２ and inevitable impurities; the atmosphere in (b) contains H with a volume content of at least 1.5%, and contains water vapor with a volume content of 0.2%, and the balance is N ２ and inevitable impurities, and the dew point is -25 to 10°C. A method for manufacturing a 120-kilogram-class ultra-high-strength galvanized steel sheet according to claim 10 ２ and inevitable impurities, and the dew point is -25 to 10°C. A method for manufacturing a 120-kilogram-class ultra-high-strength galvanized steel sheet according to claim 10 ２ and inevitable impurities, and the dew point is -25 to 10°C. A method for manufacturing a 120-kilogram-class ultra-high-strength galvanized steel sheet according to claim 10

13. In the above step (2), the heating temperature is in the range of 1150 to 1300 °C, The temperature for finish rolling is A ｃ３ ~1000 °C, and The heat preservation temperature for laminar flow cooling is in the range of (A ｃ１ ±45°C), and the residence time of laminar flow cooling is 5 to 30 s. Further, it is cooled to 550 to 650°C for coiling, and the coiled steel coil is heat-preserved for 30 to 300 min in the range of (coiling temperature T Ｃ ±30°C). A method for manufacturing a 120-kilogram-class ultra-high-strength galvanized steel sheet according to claim 9, characterized by the above.

14. In the above step (2), the heating temperature is 1165 to 1270 °C; the finish rolling temperature is 885 to 945 °C; the heat preservation temperature for laminar cooling is 680 to 720 °C, and the residence time is 7 to 26 s; the coiling temperature is 550 to 645 °C, and the heat preservation time after coiling is 45 to 270 min A method for manufacturing a 120-kilogram-class ultra-high-strength galvanized steel sheet according to claim 13, characterized by the above.

15. In the step (5), when the galvanized steel sheet is a hot-dip galvanized steel sheet, after taking out the steel sheet having a galvanized layer from the zinc pot, it is cooled to room temperature; when the galvanized steel sheet is a hot-dip zinc-iron alloy galvanized steel sheet, after taking out the steel sheet having a hot-dip zinc-iron alloy galvanized layer from the zinc pot, it is heat-insulated for 5 to 60 s in the range of (zinc pot temperature T ＺＰ - 20°C) to (zinc pot temperature T ＺＰ + 35°C) for alloying and then cooled to room temperature; preferably, T ＺＰ is 458 to 461°C, and a method for manufacturing a 120-kilogram-class ultra-high-strength galvanized steel sheet according to claim 9.

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